Radiation-induced pneumonitis (RP) is a rare complication of radioembolization with yttrium-90 ((90)Y) microspheres. The present report describes a case of RP in a patient with liver metastases from a gastrointestinal stromal tumor after radioembolization with (90)Y glass microspheres. This patient developed clinical, functional, and radiographic findings consistent with RP, with near-complete pulmonary parenchymal recovery and no clinical evidence of relapse or progressive decline in pulmonary function over a 9-month period. As clinical use of radioembolization expands, rare adverse events such as RP may become more frequent. It is essential that interventional radiologists, radiation/medical oncologists, and nuclear medicine physicians recognize this potential complication.